Malignant melanoma is a tumor whose incidence is dramatically increasing in persons with light-coloured skin in all parts of the world. Due to its resistance against traditional chemo- and radiotherapy, melanoma has been a favourite target of alternative therapies, in particular those involving immunological mechanisms. Cytokines and particularly tumor necrosis factor (TNF) have been studied as possible antitumoral agents, but also as endogenous growth or differentiation factors. Previous studies showed that melanomas could express TNF in situ and that this expression correlated to decreased lymphocyte infiltration. On the other hand, redox reagents can modulate expression of cytokines, and the thioredoxin (Trx) system is particularly known to influence expression and secretion of TNF in vitro.
The overall aim of this research was to explore immunological aspects of melanoma, particularly the role of TNF both in vitro and in vivo, as well as its possible modulation by Trx.
In the in vitro studies first we developed a novel method for obtention of monoclonal antibodies against melanoma antigens, and generated and characterized specific monoclonal antibodies against both full-length and truncated Trx. We studied the cytokine expression of a panel of normal and transformed melanocytic cells by immunofluorescence, all of which presented TNF and Trx at levels comparable to monocytic cells, and TNF-receptors (TNFR) at low but detectable levels. Melanoma cells did not secrete TNF upon stimulation in spite of its presence in the Golgi apparatus. However, melanoma cells expressed the TNF-processing enzyme TACE and were capable of cleaving transfected GFP-tagged TNF. Imaging studies point to a possible cell-cell tranfer of endogenous TNF in melanoma cells.
On the other hand, TNF and Trx expression in melanoma cell lines correlated to resistance against exogenous TNF. We studied then the in situ expression of TNF and Trx by immunohistochemistry in a group of 44 cutaneous melanoma patients. Trx expression did not correlated to survival or other clinicalpathological parameters. TNF expression significantly correlated to better survival in tumors thicker than 0,8 mm, and constituted an independent prognostic factor.
These results point to a biological role of endogenous TNF in malignant melanoma, either by constituting an autocrine/paracrine differentiation factor or by modulating communication with other cell types, particularly of the hosts immune system.